FIG. 1 shows the wiring configuration for a prior-art telephone system 10 for a residence or other building, wired with a telephone line 5. Residence telephone line 5 consists of single wire pair which connects to a junction-box 16, which in turn connects to a Public Switched Telephone Network (PSTN) 18 via a cable 17, terminating in a public switch 19, apparatus which establishes and enables telephony from one telephone to another. The term “analog telephony” herein denotes traditional analog low-frequency audio voice signals typically under 3 KHz, sometimes referred to as “POTS” (“plain old telephone service”), whereas the term “telephony” in general denotes any kind of telephone service, including digital service such as Integrated Services Digital Network (ISDN). The term “high-frequency” herein denotes any frequency substantially above such analog telephony audio frequencies, such as that used for data. ISDN typically uses frequencies not exceeding 100 Khz (typically the energy is concentrated around 40 Khz). The term “telephone line” herein denotes electrically-conducting lines which are intended primarily for the carrying and distribution of analog telephony, and includes, but is not limited to, such lines which may be pre-existing within a building and which may currently provide analog telephony service. The term “telephone device” herein denotes, without limitation, any apparatus for telephony (including both analog telephony and ISDN), as well as any device using telephony signals, such as fax, voice-modem, and so forth.
Junction box 16 is used to separate the in-home circuitry from the PSTN and is used as a test facility for troubleshooting as well as for wiring new telephone outlets in the home. A plurality of telephones 13a, 13b, and 13c connects to telephone line 5 via a plurality of telephone outlets 11a, 11b, 11c, and 11d. Each telephone outlet has a connector (often referred to as a “jack”), denoted in FIG. 1 as 12a, 12b, 12c, and 12d, respectively. Each telephone outlet may be connected to a telephone via a connector (often referred to as a “plug”), denoted in FIG. 1 (for the three telephone illustrated) as 14a, 14b, and 14c, respectively. It is also important to note that lines 5a, 5b, 5c, 5d, and 5e are electrically the same paired conductors.
There is a requirement for using the existing telephone infrastructure for both telephone and data networking. In this way, the task of establishing a new local area network in a home or other building is simplified, because there would be no additional wires to install. U.S. Pat. No. 4,766,402 to Crane (hereinafter referred to as “Crane”) teaches a way to form LAN over two-wire telephone lines, but without the telephone service.
The concept of frequency domain/division multiplexing (FDM) is well-known in the art, and provides means of splitting the bandwidth carried by a wire into a low-frequency band capable of carrying an analog telephony signal and a high-frequency band capable of carrying data communication or other signals. Such a mechanism is described for example in U.S. Pat. No. 4,785,448 to Reichert et al. (hereinafter referred to as “Reichert”). Also is widely used are xDSL systems, primarily Asymmetric Digital Subscriber Loop (ADSL) systems.
Relevant prior art in this field is also disclosed in U.S. Pat. No. 5,896,443 to Dichter (hereinafter referred to as “Dichter”). Dichter is the first to suggest a method and apparatus for applying such a technique for residence telephone wiring, enabling simultaneously carrying telephone and data communication signals. The Dichter network is illustrated in FIG. 2, which shows a network 20 serving both telephones and a local area network. Data Terminal Equipment (DTE) units 24a, 24b, and 24c are connected to the local area network via Data Communication Equipment (DCE) units 23a, 23b, and 23c, respectively. Examples of Data Communication Equipment include modems, line drivers, line receivers, and transceivers. DCE units 23a, 23b, and 23c are respectively connected to high pass filters (HPF) 22a, 22b, and 22c. The HPF's allow the DCE units access to the high-frequency band carried by telephone line 5. In a first embodiment (not shown in FIG. 2), telephones 13a, 13b, and 13c are directly connected to telephone line 5 via connectors 14a, 14b, and 14c, respectively. However, in order to avoid interference to the data network caused by the telephones, a second embodiment is suggested (shown in FIG. 2), wherein low pass filters (LPF's) 21a, 21b, and 21c are added to isolate telephones 13a, 13b, and 13c from telephone line 5. Furthermore, a low pass filter must also be connected to Junction-Box 16, in order to filter noises induced from or to the PSTN wiring 17. As is the case in FIG. 1, it is important to note that lines 5a, 5b, 5c, 5d, and 5e are electrically the same paired conductors.
However, the Dichter network suffers from degraded data communication performance, because of the following drawbacks:
1. Induced noise in the band used by the data communication network is distributed throughout the network. The telephone line within a building serves as a long antenna, receiving electromagnetic noise produced from outside the building or by local equipment such as air-conditioning systems, appliances, and so forth. Electrical noise in the frequency band used by the data communication network can be induced in the extremities of telephone line 5 (line 5c or 5a in FIG. 2) and propagated via telephone line 5 throughout the whole system. This is liable to cause errors in the data transportation.
2. The wiring media consists of a single long wire (telephone line 5). In order to ensure a proper impedance match to this transmission-line, it is necessary to install terminators at each end of telephone line 5. One of the advantages of using the telephone infrastructure for a data network, however, is to avoid replacing the internal wiring. Thus, either such terminators must be installed at additional cost, or suffer the performance problems associated with an impedance mismatch.
3. In the case where LPF 21 is not fitted to the telephones 13, each connected telephone appears as a non-terminated stub, and this is liable to cause undesirable signal reflections.
4. In one embodiment, LPF 21 is to be attached to each telephone 13. In such a configuration, an additional modification to the telephone itself is required. This further makes the implementation of such system complex and costly, and defeats the purpose of using an existing telephone line and telephone sets ‘as is’ for a data network.
5. The data communication network used in the Dichter network supports only the ‘bus’ type of data communication network, wherein all devices share the same physical media. Such topology suffers from a number of drawbacks, as described in U.S. Pat. No. 5,841,360 to the present inventor, which is incorporated by reference for all purposes as if filly set forth herein. Dichter also discloses drawbacks of the bus topology, including the need for bus mastering and logic to contend with the data packet collision problem. Topologies that are preferable to the bus topology include the Token-Ring (IEEE 803), the PSIC network according to U.S. Pat. No. 5,841,360, and other point-to-point networks known in the art (such as a serial point-to-point ‘daisy chain’ network). Such networks are in most cases superior to ‘bus’ topology systems.
The above drawbacks affect the data communication performance of the Dichter network, and therefore limit the total distance and the maximum data rate such a network can support. In addition, the Dichter network typically requires a complex and therefore costly transceiver to support the data communication system. While the Reichert network relies on a star topology and does not suffer from these drawbacks of the bus topology, the star topology also has disadvantages. First, the star topology requires a complex and costly hub module, whose capacity limits the capacity of the network. Furthermore, the star configuration requires that there exist wiring from every device on the network to a central location, where the hub module is situated. This may be impractical and/or expensive to achieve, especially in the case where the wiring of an existing telephone system is to be utilized. The Reichert network is intended for use only in offices where a central telephone connection point already exists. Moreover, the Reichert network requires a separate telephone line for each separate telephone device, and this, too, may be impractical and/or expensive to achieve.
Although the above-mentioned prior-art networks utilize existing in-home telephone lines and feature easy installation and use without any additions or modifications to the telephone line infrastructure (wires, outlets, etc.), they require dedicated, non-standard, and complex DCE's, modems, and filters, and cannot employ standard interfaces. For example, Ethernet (such as IEEE802.3) and other standards are commonly used for personal computers communication in Local Area network (LAN) environments. With prior-art techniques, in order to support communication between computers, each computer must be equipped with an additional modem for communicating over the telephone line. Whether these additional modems are integrated into the computer (e.g. as plug-in or built-in hardware) or are furnished as external units between the computer and the telephone line, additional equipment is required. The prior-art networks therefore incur additional cost, space, installation labor, electricity, and complexity. It would therefore be desirable to provide a network which contains integral therewith the necessary standard interfaces, thereby obviating the need to provide such interfaces in the DTE's.
There is thus a widely-recognized need for, and it would be highly advantageous to have, a means for implementing a data communication network using existing telephone lines of arbitrary topology, which continues to support analog telephony, while also allowing for improved communication characteristics by supporting a point-to-point topology network.
Furthermore, there is also a need for, and it would be highly advantageous to have, a means and method for implementing such an in-house data communication network using existing telephone lines, wherein the DTE's (e.g. computers, appliances) can be interconnected solely by using standard interfaces, without the need for modifications or adding external units to the DTE's.